1. Field of the Invention
This invention relates to an illuminating apparatus, and more particularly to an illuminating apparatus for illuminating a mask or reticle carrying a circuit pattern thereon by visible or invisible photoenergy in a semiconductor printing apparatus.
2. Description of the Prior Art
In the semiconductor manufacturing process, and particularly, in the printing (transfer) step thereof, an integrated circuit pattern is transferred from a mask onto a wafer. The transfer is achieved by one of several methods, including one whereby the mask and wafer are brought into contact with each other for printing, one whereby printing is effected with the mask supported at a very minute distance from the wafer, and one whereby the pattern on the mask (what is generally called a reticle is also referred to as a mask) is projected onto the wafer through an imaging optical system and printed on the wafer.
In the latter case, it greatly affects the performance of the pattern image to illuminate the mask well and for this reason, many technical improvements in this respect have been made. Various systems are known the objects which are to eliminate irregularity of illuminance on the surface of the mask, improve the converging efficiency and set the maximum value of the angle of incidence of the light beam incident on the surface of the mask (hereinafter referred to as the converging angle) to a desired value. Of these systems, the one which has been the most popular in recent years is of the type in which a plurality of secondary light sources arranged on a plane are formed from an original light source by the use of a prism or a multilens and the multiple light beams from the secondary light sources are applied to the surface of the mask by means of a converging collimator lens. Such system is disclosed, for example, in Japanese Laid-Open patent application Nos. 103976/1975 or 129045/1975. In such system, the elimination of irregularity of illuminance has been remarkably improved because the light beams from the secondary light sources scattered about the optical axis are all superposed on the wafer. On the other hand, the use of an elliptical mirror as means for converging the light beam emitted from the light source on a multi-beam forming optical element for forming secondary light sources leads to enhanced converging efficiency and therefore, it has already been used in semiconductor printing apparatus.
However, the conventional system using secondary light sources does not provide uniform luminance distribution of the secondary light sources although it has been found to be effective in the elimination of irregularity of illuminance. Therefore, with such system, it is difficult to set the effective converging angle to a desired value. That is, assuming that the secondary light sources are seen from any point on the wafer, the secondary light source near the optical axis has a greater quantity of light than other secondary light sources and thus, the luminance differs depending on the direction in which the secondary light sources are seen. Accordingly, where the difference in quantity of light between the secondary light source near the optical axis and the secondary light sources in the marginal portion is great, the substantial converging angle depends on the secondary light source near the optical axis. Even if the secondary light sources in the marginal portion are shielded from light by a stop, it is difficult to control the converging angle and as a result, improved resolution could not be expected.
For example, in the case of a system in which printing is effected with a wafer and a mask being held in close proximity to each other, diffraction is caused by the edge of the pattern of the mask. As a result, during the exposure time, the surface of the mask is irradiated from different directions in order to suppress the influence of the side-lob (sub-peak) created in the light intensity curve of the projected image. However, it has been pointed out that in this case, the resolution greatly varies depending on the value of the converging angle, and an optimum converging angle is said to be about 3.degree. for the half-angle (the article "Lenses for IC and Printing Apparatus" in the 13th Summer Seminar papers on "Optics and IC Technique"). If, as described above, it is difficult to set the converging angle to a desired value and shape, the resolution cannot be improved.
In the case of the projection printing system, the image of a secondary light source (hereinafter referred to as the effective light source) is formed on the entrance pupil of a projection lens and it is well known that the size, shape and intensity distribution of this effective light source greatly affect the imaging performance of the projection lens. Accordingly, this system also is non-uniform in luminance distribution of the secondary light source. If it is difficult to set the effective light source to a desired size, shape and intensity distribution, the improvement of the performance will be hampered.
As described above, the luminance distribution of the secondary light source is a factor which has a reat effect in determining the resolution. Even in the conventional illuminating system, a method is conceivable in which a stop for making uniform the luminance distribution of the secondary light source is disposed adjacent to the secondary light source and only a part of the secondary light source whose luminance distribution can be regarded as uniform is used. However in such method, the loss of the quantity of light is great. Therefore in reality, such method is a compromise between resolution and converging efficiency and thus, cannot be said to be an effective method.